The present invention relates to a deflection yoke and its coil bobbin, which are disposed externally to a cathode ray tube (CRT) and which produce within the CRT a deflection magnetic field that deflects electron beams emitted by the electron gun.
Deflection yokes according to prior art have generally been made by a process in which a saddle-shaped deflection coils made by winding a conductor onto a die fit into a coil bobbin made of plastic. Ill tills case, however, it was difficult to achieve the designed winding distribution with high precision. Thus a method manufacturing a deflection coil is proposed, in which the conductor is wound directly onto a coil bobbin that is provided with guide slits (the slit-saddle winding method).
For example, FIG. 17A is a top view, FIG. 17B is a front elevation view and FIG. 17C is a bottom view of a coil bobbin according to prior art, which is capable of adopting the slit-saddle winding method. As shown in the figures, the coil bobbin 51 is provided with winding hooks 52 and 53, and has in its inner surface guide slits 54 to guide the conductor. The winding bundle of the deflection coil formed by winding the conductor onto the coil bobbin 51 is of a saddle-shape, designated by a reference numeral 55 in the lower portion of FIG. 18, and the deflection magnetic field, as shown in the upper portion of FIG. 18, has a neck side region N with a strong barrel magnetic field, a middle region M with a transition from a barrel magnetic field to a pin magnetic field, and a screen side region S with a pin magnetic field. Note that the pin magnetic field, as shown by the broken lines designated with P in the cross sectional view in FIG. 19A (taken on a plane perpendicular to the center axis AX of the coil bobbin 51), is produced when the winding bundle is present in a position close (winding angle .theta. of less than 30.degree.) to the horizontal axis H passing through the center axis AX, whereas the barrel magnetic field, as shown by the broken lines designated with B in the cross sectional view in FIG. 19B (taken on a plane perpendicular to the center axis AX), is produced when the winding bundle is present in a position distant (winding angle .theta. of 30.degree. to 90.degree.) from the horizontal axis H.
For example, Japanese Patent Kokai Publications 14402/1992 and 147546/1992 disclose a proposal shown in FIGS. 20A and 20B, in which a virtually hollow funnel-shaped coil bobbin 61 is provided with winding hooks 62 in multiple tiers. FIG. 20A is a front elevation view and FIG. 20B is a perspective view of the coil bobbin. The winding bundle 63 of the deflection coil wound onto the coil bobbin 61 is of a saddle-shape, as shown in the lower portion of FIG. 21, and the deflection magnetic field, as shown in the upper portion of FIG. 21, has a neck side region N with a strong barrel magnetic field, a middle region M with a transition from a barrel magnetic field to a pin magnetic field, and a screen side region S with a pin magnetic field.
Nevertheless, in a deflection yoke using the coil bobbin 51 shown In FIGS. 17A, 178 and 17C, a problem arises in that, since all conductor turns are wound within the flange 56 of the coil bobbin 51 so that they overlap, the degree of projection outside the winding bundle 55 (designated in FIG. 18 by a reference numeral 55a) increases and the diameter of the annular core (not shown) on the outside of the winding bundle 55 is increased. Thus the deflection magnetic field cannot be produced efficiently, and deflection sensitivity is reduced.
In the deflection yoke shown in FIGS. 20A and 20B, despite the adoption of a multiple-tier structure for the flange which reduces the degree of projection of the winding bundle 63 at the bridging portion 64, the problem arises in that, since flanges 62a of winding hooks 62 protrude outside the coil bobbin 61, the diameter of the annular core mounted outside the winding bundles 63 is large. Thus, the deflection field cannot be produced efficiently, and deflection sensitivity is reduced.
This reduction in deflection sensitivity is a particularly serious problem in television sets with a wide deflection angle, high-definition television (HDTV) and the like.
In deflecting the beam from an in-line electron gun, a further problem inverse-cross misconvergence arises with both of the deflection yokes shown in FIGS. 17A to 17C and FIGS. 20A and 20B. This is a phenomenon, whereby, the position irradiated by the blue beam is skewed as shown by broken lines in FIG. 22 and the position irradiated by the red beams is skewed as shown by solid lines in FIG. 22. Each of the positions irradiated is skewed in reverse directions with each other and in directions that are reversed at the center and the periphery of the screen.
This inverse-cross misconvergence occurs in cases when, as shown in FIG. 18 and FIG. 21, the barrel magnetic field in the neck side region N is strong, the barrel magnetic field in the middle region M is weak, and the pin magnetic field in the screen side region S is weak. It is therefore possible to reduce the inverse-cross misconvergence if the barrel magnetic field in the neck side region N can be weakened, the barrel magnetic field in the middle region M, which has a major influence on the central portion of the CRT screen, can be strengthened, and the pin magnetic field in the screen side region S, which has a major influence on convergence at the periphery of the CRT screen can be strengthened. With the coil bobbins shown in FIG. 18 and FIG. 21, however, it is impossible to make a deflection yoke capable of producing such deflection magnetic fields. And with the deflection yoke shown in FIG. 20 specifically, the inverse-cross misconvergence is particularly large since the winding bundle (designated by a reference numeral 64) that produces the barrel magnetic field is disposed up to the end of the neck side region N.